1. Field of the Invention
The invention relates to an optical system and in particular to a projection exposure apparatus for microlithography. More particularly, the invention relates to a projection exposure apparatus with rotationally asymmetrical illumination, e.g. having a slot-shaped image field.
2. Description of Related Art
An optical system of this kind is known from EP 0 823 662 A2. In that system, correction beams are directed through the projection objective parallel to the projection light. The correction radiation is absorbed by at least one optical element that is exposed to projection light. This influences the imaging characteristics of the optical element, an effect which is utilized for correction purposes.
Such a correction radiation device is very complicated to adjust and restricts the usable object field, since the correction rays are coupled into the projection optical system in the region of the optical field. Only very limited adaptation to changing correction requirements is possible with this arrangement.